Patent GB-218675-A (GB)
Title: An improved method of and apparatus for converting electric power
IPC: H02M5/00
Filed: 1924-07-04
Abstract: 218,675. Hazeltine Research Corporation, (Assignees of Hazeltine, L. A.). July 5, 1923, [Convention date]. Phase and frequency, changing; vapour and vacuum apparatus, arrangements of.-Relates to thermionic valve systems developed from that described in Specification 155,789, for converting A. C. of one frequency into A.C. of a different frequency and of a different number of phases. In illustration of the theory underlying the invention similarly constructed D.C. converters are also described : these are claimed in the Specification 245,708. In particular, a 3-phase supply for electric railways is first converted to high-voltage D.C. at sub-stations, is taken in that form through trolleys and converted on the locomotives to A.C. of higher frequency and then reconverted to polyphase current of a frequency which is low and can be controlled for the purpose of controlling the speed of the induction motors which drive the locomotives. Preferably use is made of thermionic valves in which both of the current carrying electrodes are emissive, these electrodes being heated to incandescence by radiation reaching them from the reflecting walls of the valve. A number of valve elements are mounted in a single vacuous space, and the electrons are guided by a strong magnetic field. Each converter comprises a number of such valva elements which operate successively at a frequency and in an order controlled in most cases by external controller, which may comprise the mechanically-driven commutator system described below. The system is designed to prevent waste of power in the valves by employing wave forms for voltage and current such that the potential drop across a valve is low when the current through it is high, the current curves being of square form. Harmonics so generated are destroyed before reaching the output circle. Valve systems. Specification 155,789 describes a system wherein the plates of a pair of valves receive high tension from a generator working at constant current and voltage, the circuits being such that a purely sinusoidal supply is delivered on the output side, while internal losses in the valves are kept low. This necessitates the use of a high series impedance (inductance) for absorbing the voltage harmonics and a high shunt admittance (condenser) for absorbing the current harmonics, such harmonics representing the difference between the sinusoidal output and the stepped rectilinear input of the valves. The frequency is controlled by the constants of the circuits. The present Specification first describes a system in which three such apparatus are connected in delta or three valves are connected in star to give threephase output, the oscillations being generated in all the above cases by a grid-plate coupling. Six valves 1 - - 6, Fig. 4<a>, may be combined in a single structure, and a large number of examples are given of the way in which such valves may be connected for various types of conversion. In a controller described below negative charges are imparted to grids in the valves under the control of a mechanically driven commutator. In the case of Fig. 4<a>, a D.C. generator is connected at M, M<1>, and the current harmonics are absorbed by three admittances C, C<1>, C<11>, while the voltage harmonics are, absorbed by three impedances L, L<1>, L<11>, so arranged as to connect the valves in three groups of two. Alternatively, there may be two such impedances and two groups of three valves, or the impedances may be lumped' in one coil. The next Figures show various groupings of valves in which, instead of a plate and filament, there are two incandescent plates, either of which can be admissive under the control of a grid or the like (the control not being shown). According to the. nature of the grouping a valve may be " circuit-opening," when it interrupts the current for one-sixth of a cycle in its turn ; or " circuit-closing" when it passes current for onesixth of a cycle. In Fig. 8 the valves are circuit-closing. D.C. is supplied at M, M<1>, and three-phase delivered at P<1>, P<2>, P<3>, the impedance-coils L and transformer primaries T being coupled as shown. The grouping of the valves may be varied in several ways : each may have an independent D.C. generator with shunt condenser; they may be in series and separated by condensers, instead of being in parallel, as shown in Fig. 8; in three parallel groups of two series valves, or two series groups of three parallel valves, or three groups of two; and the impedance coils may be independent, coupled in two or threes or all together, or lumped in one coil. Some of the harmonics generated by individual valves cancel one another in their jcint effect; the theory for this and for the cases described below is given in the Specification. Fig. 17<a> exemplifies a system of the circuit-opening type, the harmonic-absorbing impedance Lz being located on the 3-phase side while the corresponding admittance Cy is on the D.C. side, the pair Lz, Cz being tuned to the A.C. frequency. Harmonics are also neutralized in the tertiary circuits 61 which associate the valves into three groups of two. Alternatively there may be two groups of three or a single group of six. Instead of the tertiary circuits, common magnetic cores may be provided so as to link the coils in groups : and these cores may be constructed like those of induction regulators, the primary and secondary windings being arranged in large slots whose openings come together. In Fig. 27 (circuit-opening type) the tertiary circuit 61 groups the valves into a single group: in alternative arrangements there may be two or three groups. Again, the six valves are all in parallel with respect to the D.C. mains M, M<1>: hut various series and parallel groupings may be adopted, as described in connection with Fig. 8. Fig. 36 shows an arrangement of the circuit-closing type for converting from 3-phase at one frequency to 4-phase at another frequency, still under external control which is not illustrated in that Figure. The side shown as 3-phase, which has the impedances Lz for absorbing the voltage harmonics, is spoken of as the " impedance side," white the 4-phase with admittances Cy is called the " admittance side." A number of variations are described and illustrated : thus the transformers may be replaced bv direct connections on either or both sides, and the valves may be in series instead of parallel with respect to either side, or the valves may be sub-grouped. Fig. 48 shows a 3-phase-4-phase converter of the circuit-opening type, the frequencies on the two sides being different and determined by the external control (see below). The impedance side is shown as 3-phase and the admittance side as 4-phase. In the variations described and, illustrated, the impedance may (as in Fig. 48) be made up of the leakage impedance of the transformers in series with condensers Cz, the transformers may be omitted on one or both sides, or the valves may be variously grouped in series and parallel with respect to each of the two sides of the system. Application to electric railways. High tension direct current (e.g. 10000 volts) is taken from the trolly 300, Fig. 54, and converted to 6- phase current of moderately high frequency (a few hundred or thousand cycles per second) in the valve system 301, transformed at 302, and converted to 3-phase low-frequency in the valve system 303. This low frequency can be varied so as to control the speed of the induction motors 305. The system 301 operates like. that of Fig. 17<a>, and in the system 303 (which is of the circuit-closing type, each valve being closed for 1/3 of a cycle), the input is the impedance side and the output is the admittance side, the transformer 302 providing impedance for both systems. The D.C. voltage is divided up by two series or parallel condensers Cy, which also exclude harmonics from the trolly wires. A modification is described and illustrated in which the system 303 is replaced by one in which each valve is open for i a control cycle. External control for valve systems. The sequence in which the valves in the above 6- valve systems are opened and closed, and the frequency and duration of the operations, are controlled by means exemplified by the 12-valve D.C.-six-phase converter shown in Fig. 57<a>, corresponding to 301 of Fig. 54, the D.C. mains being at M, M<1>, and the neutral point of the 6-phase output at N<1>. The valves are of the type described below, having grids which control the emission from electrodes which are all incandescent and may function as anode or cathode as required. In order that a valve may open and close abruptly it is necessary to charge and discharge its grid suddenly with a heavy charge. This charge is therefore taken from a reservoir condenser C<1>r under the supervision of a control valve 401, which is normally blocked owing to the action of the rectifier 441 but whose grid is intermittently discharged by the commutator 411 so as to allow current to pass. In order that the charge from the condenser C<1>r may be applied to the right pair of grid's such as G4, G<1>4, at any instant, a second commutator 421 is provided on the same shaft as 411. The two long segments of this are connected permanently to a rectifier 431 which maintains one of them positive and the other negative with respect to the neutral point N' of the converter, while the two small segments are connected to opposite sides of the reservoir condenser C<1>r. The effect is that the grids G4 &c., which are connected in pairs to the six brushes 461 - - 466 through biassing rectifiers such as B, receive a sudden charge or discharge at the right instants from the condenser 401 through the short segments, while at intermediate times they receive a steady potential from the rectifier 431 through the long segments. The recharging of the condenser C<1>r is assisted by the intermittent bridging of the gaps between l